Cities have a memory and interact with their neighbours

Do cities have a memory of their own and do they also ‘interact’ with other cities? Yes, says a new study.

The researchers have developed some algorithms which reveal that what happens in a given moment in a city on a demographic level depends on what happened in previous years, as well as the presence of other large cities nearby.

“We can say that the urban systems have an inertia or memory of their past”, said lead study author Alberto Hernando from the Lausanne Federal Institute of Technology (EPFL), Switzerland.

“It may sound obvious, but this implies that a decision as personal as whether to move house or emigrate also depends on how many people did the same the year before independently, people that in reality you have never even met,” Hernando added.

In large cities, population evolves depending on two factors: what ‘reminds’ them of their recent past and the existence of other urban areas around them.

If all the individual decisions of inhabitants in a city are analysed together as a set, some demographic patterns appear or a ‘collective coherence’ which can be predicted.

The researchers have applied their algorithms to cities in Spain and the US.

In the first case they used demographic data from the Spanish National Institute for Statistics (INE) for the period 1900-2011, and in the latter case, registers from the US Census Office between 1830 and the year 2000.

The results show that Spanish cities with more than 10,000 inhabitants have a medium-term memory of 15 years.

However, according to the algorithms, the memory of cities in the US lasts for 25 years.

It has also been verified that events such as civil war or the Great Depression in 1929 have shaped that memory, generating a kind of ‘post-traumatic amnesia’, which remains etched on its population for two-and-a-half more decades.

“Understanding society as a collective set of which we all form part and that behaves as a coherent entity subject to predictable rules, means that economic downturns do not only affect the individual directly harmed by the crisis, but all collective behaviour,” explained Hernando.

The other result from the research is that the city’s growth is also determined by the way in which its neighbours develop.

The typical interaction distance is 80 km in Spanish cities and 200 km for those in the US.

“The cities are not individual objects but form a part of more global network and their future is linked to their surrounding one,” said the researcher.

“It means that to make projections for the future of a city, you specifically need to know what will happen to the neighbouring cities as well,” they said.

The findings were published in the ‘Journal of The Royal Society Interface’. IANS